The systems of projection of panoramic images conventionally comprise a panoramic projection optical device comprising an image generation device, and a projection surface facing the optical device. The projection optical devices are generally designed to project on the projection surface a panoramic image generally having a field of view greater than 140°, while keeping a constant distribution of the pixels in the projected image. The projection optical devices consist for example of a digital projector comprising a plane active matrix for image generation.
The panoramic images are generally obtained using a panoramic objective of fisheye type having an angle of aperture of about 180°. Such a lens makes it possible to obtain an image having a circular shape.
FIG. 1 schematically shows the projection of a panoramic image. The figure shows a panoramic image 8 having a circular plane shape of center O and axes OX and OY, and a projection surface 2 which shape is for example substantially hemispherical. A projection optical device (not shown) allows the panoramic image 8 to be projected on the projection surface 2. In particular, the projection device projects each image point P of Cartesian coordinates (x, y) in the system OXY, on the projection surface 2 on a point P′ of Cartesian coordinates (x′, y′, z′) in a rectangular system O′X′Y′Z′. The origin O′ of the system O′X′Y′Z′ is for example located at the center of curvature of the projection surface 2, the axis O′Z′ corresponding to the optical axis of the projection optical device.
The operation of the projection optical device may be modeled by a transfer function Fp giving for each image point P(x, y), the coordinates (x′, y′, z′) of the point P′ projected on the projection surface. In prior art, the aim was to avoid the projected image from comprising distortions, unpleasant for observers. To that end, the points P of the image generated are projected on the projection surface 2 according to a most linear possible dispersion as a function of the field angle θ (angle O′Z′—O′P′) of the projected points P′.
In addition, the image generation devices usually have a plane image generation surface of rectangular shape, having a format which ratio may be 4/3 or 16/9. Consequently, the panoramic projection optical device can only project a disc inscribed in the image generation surface. The result is that all the pixels of the image generation surface located outside the inscribed disc are not projected on the projection surface.
In the panoramic projection systems with hemispherical projection screens, the projection optical device is usually located at the center of curvature of the hemispherical screen to obtain a constant distribution of the pixels of the projected image. The observers therefore cannot be also located at the center of curvature of the projection screen. Consequently, some projected points are nearer the observer than others. The result is that an observer cannot see all the pixels of the projected image with the same angular resolution.